1. Field of the Invention
The present invention relates generally to an apparatus and method for generating heat, and more particularly to a method and apparatus for providing plural controlled temperatures using multiple layers of Curie temperature materials.
2. State of the Art
Devices for providing a regulated supply of heat are known. One such device is described in commonly assigned U.S. Pat. No. 4,752,673 (Krumme) which discloses an auto-regulating, electrically shielded heater. The disclosed heater of the '673 patent provides auto-regulated heat at a single regulated temperature. Exemplary embodiments employ a non-magnetic conductive material sandwiched between two magnetically permeable materials of different Curie temperatures to provide a heating surface which can be operated at the single, regulated temperature.
FIG. 3 of the '673 patent illustrates a soldering iron which exploits a "skin effect" to provide the single, regulated temperature. As described in the '673 patent, the FIG. 3 soldering iron includes an electrically conductive, non-magnetic intermediate layer 6. The intermediate layer 6 is sandwiched between an inner magnetic layer 2 used to provide a single regulated temperature heating surface and an outer magnetic layer 4 used to provide electromagnetic shielding. The inner layer 2 is illustrated as an inner cone formed of high permeability, high resistivity, low Curie temperature material such as an NiBalFe alloy. The outer layer 4 is illustrated as an outer cone formed coaxial with and about the non-magnetic intermediate layer 6 and the inner cone 2. The outer cone 4 can be fabricated from a high permeability, low resistivity, high Curie temperature material such as low carbon steel, cobalt or nickel. A constant current AC supply 12 is connected between a center conductor 8 formed of copper and large diameter ends of the inner and outer cones 2 and 4.
In operation, alternating current from supply 12 is confined to a surface of the inner cone 2 adjacent to the return path via the conductor 8. Power dissipation is determined by the equation: P=I.sup.2 R.sub.1 where I.sup.2 is a constant K due to use of a constant current supply, and R.sub.1 is a resistance of the inner cone 2 at the frequency of the current supply. Resistance of the inner cone 2 is a function of the material resistivity and the cross-section of the inner cone 2 to which the current is confined by the skin effect. Resistance is an inverse function of cross-sectional area so that as the cross-section of the cone to which the current is confined decreases due to an increase in skin effect, the resistance of the inner cone 2 increases.
The formula for skin depth in a monolithic material is: skin depth=(5030) times the square root of (.rho./.mu.f), or 5030.sqroot.(.rho./.mu.f) centimeters where .rho. is resistivity, .mu. is magnetic permeability and f is the frequency of the constant current supply. Thus, skin depth decreases with increased frequency, while effective resistance increases.
As described at column 7, line 38 of the '673 patent, when current is initially applied to the FIG. 3 soldering iron, current is confined to the inner cone 2. The inner cone 2 is of an exemplary thickness which corresponds to one skin depth of Alloy 42 at 90 hertz (Hz). The device heats until the Curie temperature of the inner cone 2 material is attained (e.g., approximately 325.degree. C.). Once this temperature is achieved, the permeability of the inner cone 2 material decreases and current begins to spread into the intermediate layer 6 and the outer cone 4. The temperature of the material of the outer cone 4 is well below its Curie temperature and the current is therefore confined to the inner cone 2, the intermediate layer 6 and to a few skin depths of the outer cone 4 at 90 Hz.
In other words, as the Curie temperature of the inner cone 2 is attained, its magnetic permeability rapidly decreases and current spreads into the intermediate layer 6 and into the outer cone 4. Thus, the total resistance of the structure due to the presence of the highly conductive intermediate layer 6 drops dramatically to provide a high auto-regulating ratio. Further, most of the current is confined to the highly conductive intermediate layer 6 and only a small percentage penetrates the outer cone 4. The outer layer 4 is therefore only 3-5 skin depths thick to effect virtually complete shielding of the device. This permits a large auto-regulating power ratio to be realized in a relatively small device using a low frequency source (e.g., 50 Hz to 10 kHz).
U.S. Pat. No. 4,701,587 (Carter et al), U.S. Pat. No. 4,695,713 (Krumme) and U.S. Pat. No. 4,256,945 (Carter et al) also relate generally to structures which exploit an auto-regulating feature to provide single temperature heating surfaces. Despite the significant advantages realized by the methods and apparatus described in these patents, they are primarily directed to generating accurate control at a regulated fixed temperature. It would therefore be desirable to exploit advantages of these patents to achieve control at any one of plural user selected temperatures.